1. Field of the Invention
The present invention relates to a wireless power supply technique.
2. Description of the Related Art
In recent years, in order to supply electric power to an electronic device, contactless power transmission (which is also referred to as “contactless power supply” or “wireless power supply”) has begun to come into commonplace use. In order to advance the compatibility of products between manufacturers, the WPC (Wireless Power Consortium) has been organized, and the WPC has developed the Qi standard as an international standard.
FIG. 1 is a diagram showing a configuration of a wireless power supply system 100 that conforms to the Qi standard. The power supply system 100 includes a power transmission apparatus 200 (TX: power transmitter) and a power receiving apparatus 300r (RX: power receiver). The power receiving apparatus 300r is mounted on an electronic device, examples of which include cellular phone terminals, smartphones, audio players, game machines, and tablet terminals.
The power transmission apparatus 200 includes a transmission antenna 201, an inverter 204, a controller 206, and a demodulator 208. The transmission antenna 201 includes a transmission coil (primary coil) 202 and a resonance capacitor 203. The inverter 204 includes an H-bridge circuit (full-bridge circuit) or otherwise a half-bridge circuit. The inverter 204 applies a driving signal S1, specifically, in the form of a pulse signal, to the transmission coil 202. This provides a driving current flowing through the transmission coil 202, which generates an electric power signal S2 at the transmission coil 202 in the form of an electromagnetic signal. The controller 206 integrally controls the overall operation of the power transmission apparatus 200. Specifically, the controller 206 controls the switching frequency of the inverter 204 or otherwise the duty ratio or the phase of the switching of the inverter 204 so as to adjust the electric power to be transmitted.
In the Qi standard, a protocol is defined for communication between the power transmission apparatus 200 and the power receiving apparatus 300r, which enables control data S3 to be transmitted from the power receiving apparatus 300r to the power transmission apparatus 200. The control data S3 is transmitted from a reception coil 302 (secondary coil) to the transmission coil 202 in the form of an AM (Amplitude Modulation) modulated signal using backscatter modulation. The control data S3 includes electric power control data (which will also be referred to as a “packet”) which indicates an amount of electric power to be supplied to the power receiving apparatus 300r, and data which indicates the particular information for identifying the power receiving apparatus 300r. The demodulator 208 demodulates the control data S3 included in the current or otherwise the voltage applied to the transmission coil 202. The controller 206 controls the inverter 204 based on the power control data included in the control data S3 thus demodulated.
The power receiving apparatus 300r includes the reception coil 302, a rectifier circuit 304, a capacitor 306, a modulator 308, a controller 312, a charger circuit 314, and a demodulator 320. The reception coil 302 receives the electric power signal S2 from the transmission coil 202, and transmits the control data S3 to the transmission coil 202. The rectifier circuit 304 and the capacitor 306 rectify and smooth a current S4 induced at the reception coil 302 according to the electric power signal S2, thereby converting the current S4 into a DC voltage. The charger circuit 314 charges a secondary battery 102 using electric power supplied from the power transmission apparatus 200.
The controller 312 monitors the amount of electric power received by the power receiving apparatus 300r. The controller 312 generates power control data (a control error value) that indicates electric power to be supplied, based on the monitored value. The modulator 308 modulates the control data S3 including the power control data so as to modulate the coil current that flows through the reception coil 302, thereby modulating the coil current and the coil voltage applied to the transmission coil 202.
Furthermore, the Qi standard allows control data S5 to be transmitted from the power transmission apparatus 200 to the power receiving apparatus 300r. The control data S5 is superimposed on the electric power signal S2 using the FSK (Frequency Shift Keying) method, and transmitted from the transmission coil 202 to the reception coil 302. The control data S5 may include an acknowledgement (ACK) signal that gives notice of reception of the control data S3 and a negative acknowledgement (NACK) signal that gives notice that the control data S3 has not been received.
An FSK modulator 220 is built into the controller 206. The FSK modulator 220 changes the switching frequency of the inverter 204 according to the data to be transmitted. The demodulator 320 arranged on the power receiving apparatus 300r side demodulates the FSK-modulated control data S5.
FIG. 2 is a circuit diagram showing the rectifier circuit 304 and the demodulator 320 investigated by the present inventors. The rectifier circuit 304 is configured as a so-called synchronous rectifier circuit (which is also referred to as the “synchronous detection circuit”) including an H-bridge circuit 330, a driver 332, a first comparator 334, a second comparator 336, and a logic circuit 338. The H-bridge circuit 330 includes transistors M1 through M4 and rectifier diodes D1 through D4.
A reception antenna 301 is connected to input terminals AC1 and AC2 of the synchronous rectifier circuit 304. An AC current IAC (S4 in FIG. 1) induced due to the electric power signal S2 flows through the reception antenna 301. The rectifier circuit 304 switches the state ϕ of the H-bridge circuit 330 at a timing at which the AC current IAC becomes zero, i.e., at a timing at which the polarity is inverted. Such a switching control operation will be referred to as “zero-current switching”. The H-bridge circuit 330 can be switched between the following four states ϕ1 through ϕ4.
A first state ϕ1 in which the first transistor M1 is on, the second transistor M2 is off, the third transistor M3 is off, and the fourth transistor M4 is on.
A second state ϕ2 in which the first transistor M1 is off, the second transistor M2 is off, the third transistor M3 is off, and the fourth transistor M4 is off.
A third state ϕ3 in which the first transistor M1 is off, the second transistor M2 is on, the third transistor M3 is on, and the fourth transistor M4 is off.
A fourth state ϕ4 in which the first transistor M1 is off, the second transistor M2 is off, the third transistor M3 is off, and the fourth transistor M4 is off.
In the second state ϕ2 and the fourth state ϕ4, the rectifier circuit 304 functions as a diode rectifier circuit.
A first comparator 334 compares a voltage VAC1 at the AC1 terminal with a zero-current detection threshold voltage VZC1. A second comparator 336 compares a voltage VAC2 at the AC2 terminal with a zero-current detection threshold voltage VZC2. The comparators 334 and 336 are each configured as a hysteresis comparator with a threshold voltage that is switched between two values, i.e., a negative voltage (e.g., −0.2 V) and a voltage in the vicinity of zero (e.g. −2 mV).
The logic circuit 338 controls the state of the H-bridge circuit 330 based on a combination of an output AC1_DET of the first comparator 334 and an output AC2_DET of the second comparator 336. The driver 332 drives the transistors M1 through M4 according to a control signal received from the logic circuit 338. It should be noted that the entire configuration and operation of the rectifier circuit 304 described with reference to FIG. 2 cannot be recognized as a known technique.
FIG. 3 is an operation waveform diagram showing the operation of the rectifier circuit 304. It should be noted that the vertical axis and the horizontal axis shown in the waveform diagrams and the time charts in the present specification are expanded or reduced as appropriate for ease of understanding. Also, each waveform shown in the drawing is simplified or exaggerated for emphasis for ease of understanding.
When the voltage VAC1 at the AC1 terminal becomes higher than −2 mV, the AC1_DET signal is switched to the high level. When the voltage VAC1 becomes lower than −0.2 V, the AC1_DET signal is switched to the low level. In the same way, when the voltage VAC2 at the AC2 terminal becomes higher than −2 mV, the AC2_DET signal is switched to the high level. When the voltage VAC2 becomes lower than −0.2 V, the AC2_DET signal is switched to the low level. The logic circuit 338 switches the state of the H-bridge circuit 330 between the first state ϕ1 through the fourth state ϕ4 based on the AC1_DET signal and the AC2_DET signal. A predetermined delay may be provided between the level transitions in the AC1_DET signal and the AC2_DET signal and the state transition.
Returning to FIG. 2, description will be made regarding the demodulator 320. The AC2_DET signal has the same frequency as that of the AC current IAC, i.e., the frequency of the electric power signal S2. Accordingly, the demodulator 320 counts the number of cycles of the AC2_DET signal so as to detect the frequency of the AC2_DET signal, and performs FSK demodulation. There is symmetry between the AC1 terminal and the AC2 terminal. Thus, the demodulator 320 may perform the FSK demodulation based on the AC1_DET signal.
However, the demodulator 320 shown in FIG. 2 has the following problem. FIGS. 4A and 4B are operation waveform diagrams each showing the operations of the rectifier circuit 304 and the demodulator 320. An FSK_CLK_ID signal is configured as a signal obtained by retiming the AC1_DET signal using an internal clock, and by masking short chattering. The FSK_CLK_ID signal can be recognized as being substantially the same as the AC1_DET signal.
In the wireless power supply operation, in addition to the FSK modulation for communication, the power transmission frequency, the switching duty ratio, or the switching phase is changed in order to adjust the transmitted electric power. As a result, in some cases, large ringing occurs in the voltage VAC1 at the AC1 terminal and in the voltage VAC2 at the AC2 terminal in the second state ϕ2 or in the fourth state ϕ4 in which the H-bridge circuit functions as a diode rectifier circuit.
As shown in FIG. 4B, when the amplitude of such ringing is large, the voltage VAC1 (VAC2) crosses the threshold voltage VZC regardless of the zero-crossing of the AC current IAC, which changes the level of the AC1_DET signal (or AC2_DET signal). This leads to a problem in that the frequency of the electric power signal S2 does not match the frequency of the AC1_DET1 signal (FSK_CLK_ID signal). This leads to a problem of degraded communication quality and degraded communication stability, and a problem of a degraded bit error rate.